This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Geothermal power plants usually have fairly low thermal efficiencies relative to solar thermal plants and most other power plants, because of the lower-temperature fluids produced from most geothermal reservoirs. Even a stream produced from a highly efficient geothermal resource will normally flash steam with a temperature less than 450° F. (232° C.). An optimized steam Rankine-cycle power plant utilizing steam flashed from produced geothermal brine will typically enjoy a thermal efficiency of 25% or less, and such efficiency only applies to heat available in the flashed steam, typically less than a quarter of the total mass of the produced geothermal fluids. For this reason, many geothermal resources that might otherwise be considered potential sites for geothermal electric power production do not have sufficiently high thermal efficiency to result in an economically attractive project. Thus, many geothermal and hydrothermal reservoirs are not developed for electric power generation. The thermal energy otherwise available in such resources remains inaccessible from an economic standpoint and thus remains untapped.
Typically, geothermal power plants are fairly small, with the majority less than about 100 MW in generating capacity, as a result of reservoir and other limitations. Despite current limitations in generating capacity, which result from a combination of the limitations of current methods, commercial considerations, and reservoir characteristics, many geothermal reservoirs contain a very large amount of thermal energy that could be extracted if the combination of technological and commercial considerations allowed, especially over a long period of time. The available geothermal heat per square mile of geothermal field associated with a 50° F. (28° C.) temperature change within the field (fluids and rock included) is believed to be over 190 trillion BTU per square mile. In an eight-square-mile geothermal structure, the available thermal energy could reach about 1500 trillion BTU, the heat-equivalent of approximately 1.5 TCF of natural gas. Thus, geothermal heat sources are potentially very large energy sources, if they can be tapped and utilized efficiently. Unfortunately, a substantial majority of these sources do not have the requisite temperatures and hydrothermal flows needed to economically sustain a geothermal power plant over a period of time sufficient to make such a project economically attractive. Thus, methods to efficiently access a greater portion of the immense thermal energy within a broad range of geothermal reservoirs would substantially increase society's ability to harness geothermal resources for electric power generation.
There have been attempts to recover heat energy from geothermal heated rock formations that do not contain significant quantities of water. Generally these formations are called hot dry rock (HDR) formations to distinguish these sources of geothermal heat from traditional geothermal heat sources such as hydrothermal fields or dry steam fields. The methods for recovery of geothermal heat from HDR formations typically involves drilling a well into the rock formation, fracturing the formation and mapping the fractured structure, then drilling and completing a second well into the fractured zone of the rock formation. A fluid, typically water or brine, is injected into the first well and migrates through the rock fractures to the second well. The fluid will absorb heat from the HDR and the heated fluid will be produced in the second well and typically used to provide heat to a geothermal power plant. The methods for drilling, fracturing, treating, and completing the set of wells for injecting and recovering heated fluid are diverse and extensive and significant current effort to develop these methods are underway. Collectively these methods are typically called ‘engineered geothermal systems’ (EGS).
The temperatures of HDR formations will generally increase with depth, and although geothermal heated rock formations could be reached by deep drilling, in certain geographical regions there are locations where higher temperature gradients exist. In these location and higher temperature geothermal rock formations are more accessible. The United States Department of Energy contributed funding for a number of initial EGS projects to test methods to recover geothermal energy from HDR formations in anomalous geological structures containing rock formations at a depth accessible with drilled wells without incurring extraordinary drilling costs. For the United States mainland for example, certain regions in the west have rock formation temperatures believed to be suitable for EGS projects to recover geothermal energy from HDR at depths that can be reached with wells drilled to a depth of approximately 20,000 feet. Some locations should encounter rock formations with suitable temperatures at less than approximately 15,000 feet.
Anderson (1978) attempted to increase the overall efficiency of a geothermal power plant by segregating higher-temperature wells that produce more steam into a high-temperature gathering system and collecting lower-temperature geothermal fluids in a separate gathering system. In the geothermal electric power plant, the higher-temperature thermal energy is transferred by heat exchange into a dual power-fluid cycle, which improves the capability of the plant to efficiently generate electric power. Unfortunately, sizable geothermal reservoirs that are suitable for the segregation process of Anderson are generally rare, resulting in limited opportunities for the application of this process.
There have been methods described that very high temperature aqueous solutions at supercritical conditions could be used to enhance oil recovery and to create synthetic geothermal reservoirs in oil fields. Specifically, Meksvanh et al. (2006) describes a method for injecting a supercritical brine into porous or permeable geologic structures (e.g., sedimentary rock formations) for the purpose of enhancing oil recovery from oil fields. The resulting synthetic reservoirs can subsequently be used for thermal storage and electricity production. The Meksvanh method is understood to use solar concentrators to heat reservoir brine directly to temperatures exceeding both the critical temperature and the critical pressure of the brine, which should exceed the supercritical temperature and pressure of water (374° C. and 22 MPa, or 705.4° F. and 3204 psia).
There have also been attempts to use solar energy to “augment” geothermal energy by heating geothermal fluids after they are produced from a reservoir. Rappoport (1978) used heat-transfer fluids to collect geothermal heat from remote wells, then uses solar collectors to replenish heat lost from these streams in transit and to add heat to the heat-transfer fluid before utilizing the heat in a centralized geothermal power plant. There have been attempts to evaluate and develop hybrid solar geothermal energy electric power generation systems. In these processes, the radiant energy from solar concentrators is absorbed directly into the fluids that contain the geothermal sourced heat and these fluids are used for power generation.
Various types of solar thermal electric power generation plants either already exist commercially or are in late developmental stages. These plants collect and concentrate solar energy (energy contained in sunlight) and convert the solar energy to thermal energy (heat). The thermal energy is then used to generate electric power.
Even in geographic locations that enjoy substantial, strong sunlight and relatively clear weather year-round, the available sunlight is often not sufficient to generate enough electricity to fully utilize, and maximize the economic investment in, a solar thermal plant. For example, solar thermal plants that lack thermal energy storage capabilities cannot generate electricity during nighttime or on overcast days. In addition, the number of hours of daylight are defined and constrained by season.
Some of these limitations can be overcome or lessened by storing thermal energy produced when sunlight is sufficient and recovering it to generate electricity when sunlight is unavailable or insufficient. The degree to which these limitations can be overcome or lessened, and the degree to which the overall utilization of the plant can be expanded, depend primarily on the amount of thermal storage available to the plant and the size of the solar energy collection field relative to the plant's electricity-generating capacity. Most of the thermal storage approaches that have been commercialized to date involve limited capacities that facilitate storage of thermal energy sufficient to operate the generators for about four to six hours. However, some studies suggest that it may be possible to store thermal energy for up to about 16 hours. This storage capacity allows a solar thermal plant to generate electricity into the late afternoon and evening hours or, at most, overnight, following a day of sufficient sunlight. However, it generally does not allow a plant to store thermal energy during a sunny season for electricity production during a less sunny season or during successive overcast days, or to continue operation during successive overcast days even in a season of normally strong sunlight.
Current methods of short-term thermal energy storage (TES) typically include steam accumulators, pressurized hot-water tanks, hot oil/rock storage vessels, solid media storage (usually concrete or ceramics), and molten salt. These methods become costly when used to store more than a few hours worth of the heat needed for medium-size or larger electric power plants. In addition, none of these methods adequately addresses long-term, seasonal storage needs.
Accordingly, it would be desirable to provide an improved system and method for geothermal production that overcomes the drawbacks and limitations of the known systems.